This invention relates to electronic test equipment; and more particularly, to test equipment for debugging digital logic systems. Such systems typically are comprised of thousands of digital logic circuits which are interconnected via thousands of leads. One example of such a system is a digital computer.
The logic circuits in digital computers and other logic systems consist essentially of AND gates, OR gates, NAND gates, NOR gates, and INVERTERS. These gates may be implemented in a wide variety of ways, such as with T.sup.2 L circuits, ECL circuits, etc. However, regardless of their particular implementation, they are all characterized under normal operating conditions as receiving input signals and generating output signals of only two voltage levels.
But when a logic gate becomes defective or when an interconnection between two logic gates is made improperly, then abnormal voltages are generated in the system. For example, abnormal voltages of 1.2 volts or 1.6 volts are often generated by such defects in T.sup.2 L systems whose two normal voltage levels are 0.4 volts and 3.8 volts. These abnormal voltages cause the system to malfunction; and thus test equipment is required to locate the defect.
In the past, scopes and analog voltmeters have been used to locate these defects. But such equipment has been found unsatisfactory because the voltage levels that are sensed are displayed in a visual fashion. This is a problem because one must continually move his head back and forth from the node that is being probed to the visual display.
To fully appreciate this problem, it should be reiterated that typical logic systems contain thousands of logic gates and interconnections. And to track down an abnormal voltage level in such a system generally requires many of those gates to be probed. Thus, the back and forth head movement is not only an aggravation; but it also makes it very difficult to keep the probe on a particular node. Often, the probe will slip off the node; and with compact integrated circuits, each particular node is difficult to find.
Therefore, it is a primary object of the invention to provide improved test equipment.
Still another more specific object of the invention is to provide a meter which indicates the level of analog voltages in a non-visual fashion.